1. Field of the Invention
The present invention relates to an automatic transmission which changes the speed of rotation of an input shaft among multiple speeds to transmit the rotation to an output shaft through a single planetary gear set and a compound planetary gear set, both provided between the input shaft and the output shaft.
2. Description of the Related Art
For automobile vehicle, there are known automatic transmissions in which elements of a single planetary gear set coupled to an input shaft and elements of a compound planetary gear set are coupled through plural clutches or fixed through brakes to change the speed of rotation of the input shaft and transmit the rotation to an output shaft. For example, an automatic transmission described in Japanese Patent Application Publication No. 2006-161867 changes the speed of rotation of the input shaft in a range of 6 forward speeds and 1 reverse speed to output the rotation to the output shaft. In this automatic transmission, the input shaft and the output shaft are axially supported by a transmission case, and an intermediate member is axially supported on the input shaft rotatably relative to the input shaft. One end of an intermediate shaft is fitted into and coaxially supported by the intermediate member rotatably relative thereto, and the other end of the intermediate shaft is coaxially supported by the output shaft rotatably relative thereto. The single planetary gear set includes an input element (carrier) directly connected to the input shaft, a fixed element (sun gear) fixed to the transmission case, and a speed increasing element (ring gear) which increases the speed of rotation of the input shaft. The speed increasing element is directly connected to the intermediate member. The compound planetary gear set includes a first rotating element capable of being coupled to the input element through a third clutch and being fixed through a first brake, a second rotating element capable of being coupled to the intermediate shaft through a second clutch and being fixed through a second brake, a third rotating element directly connected to the output shaft, and a fourth rotating element capable of being coupled to the input element through a first clutch.
However, in the conventional structure, since the second clutch is provided at the side of the compound planetary gear set opposite to the single planetary gear set in the axial direction, it is necessary to provide the intermediate shaft capable of coaxially rotating relative to the input shaft. The intermediate member also needs to be provided to couple the ring gear as the speed increasing element of the single planetary gear set to the intermediate shaft along the axis of the input shaft. Thus, the numbers of the torque transmitting elements arranged in the axial direction of the input shaft are increased. In this structure, when an output rotation with a relatively low rotational speed and high torque is input, especially in a diesel engine, allocated torque for each part of the automatic transmission becomes large. In this case, it is necessary to use clutches and brakes with high torque capacities, resulting in increasing the size of the automatic transmission and hence making it difficult to mount the automatic transmission on a vehicle, in other words, deteriorating the mountability thereof on a vehicle.
Further, the first to third clutches are generally designed in such a manner that a piston presses a friction plate through working oil to couple two members of the 3 clutches. Therefore, a structure can be considered, in which a piston chamber of the first clutch is provided on the fourth rotating element side to make the allocated torque small so that clutches and brakes with low torque capacities can be used. In this structure, however, since the fourth rotating element rotates fast at the sixth gear speed, excessive centrifugal pressure occurs in the piston chamber, which is possible to impede the smooth action of the first clutch.